Friction material

ABSTRACT

Improved friction materials comprising a synthetic metal oxide fibre and a binder. The most useful fibres are polycrystalline alumina, alumina/silica or zirconia, and especially those made by fibrising an aqueous solution of a compound of the metal containing a water-soluble organic polymer and having a viscosity above 1 poise, drying the fibre and heating. Fibres of other oxides or mixed oxides may be used. The materials may include additives (lubricants, fillers, and metals) and are useful in brake units, in which they have advantages over conventional (cotton and asbestos) materials in their heat stability and reduced tendency to &#39;&#39;&#39;&#39;fade&#39;&#39;&#39;&#39; in service.

United States Patent 11 1 1111 3,891,595 Birchall June 24, 1975 [54]FRICTION MATERIAL 3,385,915 5/1968 Hamling 106 51 3,503,765 3/1970 Blazel06/65 [75] Inventor: James Derek Rumor", 3,550,739 12 1970 Wiltseyl06/36 England 3,598,677 8/l97l Bergmeister et al .1 l06/36 x [73]Assignee: Imperial Chemical Industries Limit d, L d E l d PrimaryExaminer-Donald E. Czaja Assistant Examiner-S. M. Person [22] F'led:1973 Attorney, Agent, or FirrnCushman, Darby & [2]] Appl. No.1 391,515Cushman [30] Foreign Application Priority Data [57] ABSTRACT Aug. 30,1972 United Kingdom 40189/72 p o d f ction materials comprisin asynthetic metal oxide fibre and a binder. The most useful fibres [52]US. Cl. 260/38; 106/38; 260/D1G. 39 are polycrystalline alumina,alumina/silica or zirconia, [51] Int. Cl C08g 51/04 n p i lly hos ma eby fibrising an aqueous so- [58] Field of Search 260/DIG. 39, 38; 106/36lulion f a mpo nd of the metal containing a watersoluble organic polymerand having a viscosity above [56] References Cit d 1 poise, drying thefibre and heating. Fibres of other UNITED STATES PATENTS oxides or mixedoxides may be used. The materials may include additives (lubricants,fillers, and metals) x32; 232;? and are useful in brake units, in whichthey have ad- 2:267:9l3 12/1941 Halsteadt: Ill III. 106 36 vantages Overand asbestos) 2369502 2/1945 Walker I I n 260/1316. 39 terials in theirheat stability and reduced tendency to 2,915,475 12 1959 Bugosh 106/36in s 2,973,336 2/l96l Delaplace 260/DIG. 39 3,384,578 5/1968 106/65Clam, Drawmgs 1 FRICTION MATERIAL This invention relates to frictionmaterials and particularly, though not exclusively, to frictionmaterials usable for brake and clutch surfaces.

Conventional friction materials consist normally of cotton or asbestosfibres impregnated with a resin and made up into a variety of forms suchas segments, cones, discs, rollers and so forth, which forms may berigid or flexible. Cotton is used largely for light duties involvingrelatively low temperatures at the friction interface, whereas asbestosis used for heavier duties and consequent higher temperatures. Asinterface temperature increases, however, the friction coefficientfalls, leading to brake fade." In addition, a proportion of the asbestosmay be liberated from the friction material as the latter wears and canproduce a significant atmospheric pollution problem, for example in thecase of automobiles used in towns.

According to the present invention there is provided a friction materialcomprising a synthetic metal oxide fibre and a binder. Syntheticinorganic fibres can be produced with controlled physical properties,which is an advantage in making friction materials with reproducibleproperties.

The fibre may be used in unwoven or woven form for example loose staple,papers, felts, mats or cloths.

We have found that synthetic inorganic fibres comprising one or morepolycrystalline refractory metal oxides are especially suitable, astheir physical properties make them capable of withstanding extremes oftemperature conditions without serious loss of strength and they can bereadily produced at the fine diameters and shot-free quality preferred.Vitreous fibres tend to devitrify under severe conditions of temperatureor under the influence of other aggressive conditions. We prefer to usesingle metal oxides, double metal oxides or mixtures of oxides; theseinclude thoria; urania; rare earth oxides; yttria; titania; chromia;magnesia; zinc oxide; calcia; double oxides of alkaline earth oxides andalumina, zirconia or chromia, for example BaO/6Al O BaO/ZrO CaO/Cr OMgO/Al o MgO/Cr O SrO/ZrO and other double oxides, for example Al- O Weespecially prefer to use polycrystalline alumina, alumina/silica orzirconia fibres prepared for example as disclosed in our co-pendingUnited Kingdom Patent Applications Nos. 36,693/72; l2,088/72; 4,369/7land 29,909/70, as fibres thus prepared have a high coefficient offriction, are hard and have other desirable properties as hereinafterdescribed. These disclosures are incorporated herein by reference.

As disclosed in these applications alumina or zirconia fibres are formedby fibrising a composition having a viscosity of greater than I poisecomprising an aqueous solution of a metal compound for example anoxychloride, basic acetate, basic formate or nitrate of aluminium and/orzirconium, and a minor proportion of a water-soluble organic polymerespecially polyethylene oxide, polyvinyl alcohol (and, whenalumina/silica fibres are requires, a water-soluble organic siliconcompound such as polysiloxane), drying the fibre formed and heating todecompose the metal compound to oxide and to decompose the polymer.Heating in the presence of steam is often preferred.

Fibrising is preferably carried out by a blowing process which comprisesextruding the fibrising composition through one or more apertures intoat least one converging gas stream having a component of high velocityin the direction of travel of extruded composition. The dimensions andshape of the said aperture may vary widely. We prefer to use an aperturehaving at least one dimension larger than 5 microns and smaller than 500microns. The gas stream is preferably air, more preferably air atambient temperature. It is convenient to employ two streams of gas whichconverge at or near the point where the composition is extruded from theaperture; preferably the angle between the converging gas streams isfrom 30 to 60. At least part of the water in the composition is removedby the gas stream, and the rate of removal may conveniently becontrolled by mixing the gas with the water vapour, for example air at arelative humidity of greater than percent may be used. The velocity ofthe air stream may be varied over wide limits, but we prefer to usevelocities in the region of 200 to l,500 feet per second. The pressureemployed to extrude the composition through the apertures will depend onthe viscosity of the composition and on the desired rate of extrusion.We find that pressures from 16 to pounds per square inch absolute areconvenient for compositions having viscosities up to about 100 poise.

The average diameter and diameter distribution of the fibres isimportant; fine fibre diameters and relatively narrow diameterdistributions are preferred. An average fibre diameter from 0.5 to 10microns is especially preferred; a diameter distribution which ensuresthat the fibres contain not greater than 30 percent by weight of fibresof greater diameter than 5 microns, or not greater than 20 percent bynumber of fibres of greater than 5 microns is also especially preferred.Alumina or zirconia fibres available under the Trade Mark of Saffil areespecially useful as such fibres have these properties.

Growth of the crystallites in the fibres on storage or during use ispreferably prevented, for example by the presence in the fibre of astabiliser, for example in the case of zirconia fibres a minorproportion, for example I to 10 percent of one or more of alkalineoxides, yttria, rare earth oxides, alumina, thoria or hafnia, and in thecase of alumina fibres a minor proportion of siltea.

The amount of shot in the fibres, that is, material of a non-fibrousnature, should be as low as possible; shot content less than 5 percentby weight is preferred.

Alumina fibres are especially useful in preferred embodiments of theinvention. Alumina exists in a number of phase forms and transformationof one to the other depends upon the temperature to which the alumina issubjected; while not wishing to be restricted to any particular theoryit is believed that this property of alumina gives an advantage overmany other inorganic fibres used in friction material. Thus in use, thefriction materials according to the invention are subjected to atemperature which depends upon the degree of friction produced at abraking interface. At a temperature greater than about 700Cnon-crystalline alumina is converted, at least partially, to atransitional alumina (for example "eta" alumina). At highertemperatures, for example greater than 950C, alpha" alumina is formed.The transitional and alpha aluminas consist of small crystallites, forexample of the order of 100 Angstroms, which, although hard andfriction-generating, are not gritty so that they do not accelerate thewear of the braking counter-surface. In addition, the hardness of alphaalumina is greater than that of transitional alumina which is in turnharder than non-crystalline alumina. It is believed therefore that asthe friction temperature increases the coefficient of friction betweenthe braking surface increases. Thus a braking action involving frictionmaterials according to the invention is less subject to fading.

The temperatures at which the non-crystalline alumina is converted to atransitional alumina or a transitional alumina is converted to alphaalumina may be varied within useful limits by suitable modification ofthe composition of the alumina fibres and/or the methods used to makethem. Methods used to modify the composition of alumina fibres aredisclosed in the patent applications hereinbefore referred to. Thus adesirable degree of control over the effect of temperature oncoefficient of friction may be achieved by a suitable modification ofthe alumina fibre composition or the use of suitable blends of aluminafibre.

The binder used in the friction material according to the invention ispreferably an organic binder, for example a thermosetting resin or anatural or synthetic rubber. The most preferred binder is one based on aphemoi-formaldehyde resin. Combinations of binders may also be used.

The friction materials according to the invention may comprise furthermaterials to achieve any particular desired combination of properties.Thus lubricants, for example litharge, graphite or molybdenumdisulphide; fillers, either inert or friction-augmenting, for exampleground barytes, perlite, corundum, borax or clays; metals such as zinc,copper, brass, lead or iron, conveniently in the form of wires, turningsor powders for example as scavengers may be incorporated as desired.

The relative proportions of metal oxide fibre, binder and otheringredients of the friction material are chosen as normally in the artto give the properties required for the duty to be performed. Normallythe binder is present in the range to 35 percent by weight of the total,the metal oxide fibre in the range 40 to 85 percent by weight of thetotal and other ingredients from 0.1 to 25 percent by weight of thetotal.

The friction materials according to the invention are prepared by anyconvenient process known in the art. The fibre is incorporated into thebinder and the mixture shaped into the desired end form for example bydry processing, sheeter processing or processes suitable for theincorporation of woven fibre into the binder. Wet board processing isespecially preferred for loose fibre; in this process the weighedingredients except the binder are dispersed for example by adding to abeater or hydrapulper with a quantity of water. The wet mixed pulp isthen further diluted and felted for example by feeding to a paper-makingmachine to be formed into a blanket of suitable thickness. This feltedproduct is then saturated in a liquid binder, for example a solution ofa phenolic resin in an evaporable solvent, shaped and solidified usuallyby drying. For fibre in the form of mats, blankets and papers, thebeating stage may be omitted.

The friction materials according to the invention may be fabricated orformed into any shape suitable for the application in which it is to beused. Thus they may be formed into segments or discs for use as brakelinings or clutch facings.

The invention is illustrated by but not limited to, the followingExamples.

EXAMPLE I 30 grams of Saffil alumina fibre blanket (ex lCl Ltd.) wasshredded into approximately I cm pieces, added to about 250 ml of waterand dispersed by means ofa high speed laboratory stirrer. The dispersedfibres were separated off, dried and mixed with l0 grams of aheatsetting liquid phenolic resin (R674l ex BXL) dissolved in 100 ml ofacetone. The acetone was removed by applying vacuum. The dry mix wasthen slightly moistened with water and compressed into a small block ina hydraulic press at 4,500 pounds line pressure. The compressed blockwas dried at C and cured at l30C for several hours (e.g., over night).

Friction/wear tests were carried out on the block using the rubber weartests defined by BS.903 Part A9 (Method A) and ASTM D394.47 (Method A)using a Croydon abrasion tester. Abrasive paper (600C grade) was used onthe disc and a force of 3.6 kg (approximately equal to 1.8 kg/cm) wasused to hold the sample against the disc.

Comparative results were obtained using fine glass fibre, asbestos fibreand an alumino-silicate fibre in place of the alumina fibre in the testblock. The results obtained are shown in Table 1.

Table l Fibre Sample Coefficient of Friction Wear (11-) (ASbBSlOF lSafl'll" alumina 0.59 0.08 Fine glass 0.50 0.08 Alummo-silicate 0.46l.00 Asbestos 0.5 l l .00

EXAMPLE 2 Example I was repeated using Saffil zirconia fibre in place ofthe alumina fibre. The results were: Coefficient of Friction (p) 0.5l;wear (asbestos=l) 0.04.

EXAMPLE 3 Test blocks were made as in Examples 1 and 2 using Saffilalumina fibre or Saffil zirconia fibre, but in addition to the fibre afiller a filler was added to the wet mix of fibre and resin solution.Ground kaolin and perlite were used as fillers and the results offriction wear tests carried out as described in Example I are shown inTable 2.

What I claim is:

l. A friction material comprising from 40 to 85 percent by weight of asynthetic inorganic polycrystalline refractory metal oxide fibre havingan average fibre diameter of from 0.5 to lO microns having a shotcontent of less than 5 percent by weight,

and from l5 to 35 percent by weight of a binder.

2. A friction material as claimed in claim 1 wherein the fibre comprisesthoria, urania, a rare earth oxide, yttria, titania, chromia, magnesia,zinc oxide, calcia or a double oxide of an alkaline earth oxide andalumina, zirconia or chromia, or mixtures thereof.

3. A friction material as claimed in claim 1 wherein the fibre is apolycrystalline alumina fibre.

4. A friction material as claimed in claim 1 wherein the fibre is apolycrystalline zirconia fibre.

5. A friction material as claimed in claim 3 wherein the fibre is formedby fibrising a composition having a viscosity greater than l poisecomprising an oxychloride, basic acetate, basic formate or nitrate ofaluminium and a minor proportion of a water-soluble organic polymer,drying the fibre and heating to form the oxide of aluminium and todecompose the polymer.

6. A friction material as claimed in claim 4 wherein the fibre is formedby fibrising a composition having a viscosity greater than 1 poisecomprising an oxychloride, basic acetate, basic formate or nitrate ofzirconium and a minor proportion ofa water-soluble organic polymer,drying the fibre and heating to form the oxide of zirconium and todecompose the polymer.

7. A friction material as claimed in claim 5 wherein the water-solubleorganic polymer is polyethylene oxide, polyvinyl-alcohol orpolyvinylpyrrolidone.

8. A friction material as claimed in claim 3 wherein fibrising iseffected by extruding the composition through one or more apertures intoat least one converging gas stream having a component of high velocityin the direction of travel of the extruded composition.

9. A friction material as claimed in claim 8 wherein the gas is air at arelative humidity of greater than percent.

10. A friction material as claimed in claim 1 wherein the metal oxidefibre has a fibre diameter distribution such that the fibres contain notgreater than 30 percent by weight of fibres of greater diameter than 5microns.

11. A friction material as claimed in claim 4 wherein zirconia fibre isstabilised with a minor proportion of an alkaline earth oxide, yttria, arare earth oxide, alumina, thoria or hafnia or mixtures thereof.

12. A friction material as claimed in claim 3 wherein the alumina fibreis stabilised with a minor proportion of silica.

13. A friction material as claimed in claim 3 wherein the alumina istransitional alumina.

14. A friction material as claimed in claim 1 wherein the binder isbased on a phenol-formaldehyde resin.

15. A friction material as claimed in claim 1 comprising a lubricantselected from litharge, graphite or molybdenum disulphide.

16. A friction material as claimed in claim 1 comprising a fillerselected from ground barytes, perlite, corundum, borax or a clay.

17. A friction material as claimed in claim 1 comprising a metal in theform of wires, turnings or powders selected from zinc, copper, brass,lead or iron.

18. A process for preparing a friction material as claimed in claim Iwherein a synthetic metal oxide fibre is incorporated into a binder andthe mixture shaped into a desired end form.

19. A process as claimed in claim 18 wherein the friction material isprepared by wet board processing, comprising the steps of dispersing thefibre, felting the dispersed fibres, saturating the felted fibres in aliquid binder, shaping and solidifying.

1. A FRACTION MATERIAL COMPRISING FROM 40 TO 85 PERCENT BY WEIGHT OF ASYNTHETIC INORGANIC POLYCRYSTALLINE REFRACTOTY METAL OXIDE FIBRE HAVINGAN AVERAGE FIBRE DIAMETER OF FROM 0.5 TO 10 MICRONS HAVING A SHOTCONTENT OF LESS THAN 5 PERCENT BY WEIGHT, AND FROM 15 TO 35 PERCENT BYWEIGHT OF A BINDER.
 2. A friction material as claimed in claim 1 whereinthe fibre comprises thoria, urania, a rare earth oxide, yttria, titania,chromia, magnesia, zinc oxide, calcia or a double oxide of an alkalineearth oxide and alumina, zirconia or chromia, or mixtures thereof.
 3. Afriction material as claimed in claim 1 wherein the fibre is apolycrystalline alumina fibre.
 4. A friction material as claimed inclaim 1 wherein the fibre is a polycrystalline zirconia fibre.
 5. Afriction mAterial as claimed in claim 3 wherein the fibre is formed byfibrising a composition having a viscosity greater than 1 poisecomprising an oxychloride, basic acetate, basic formate or nitrate ofaluminium and a minor proportion of a water-soluble organic polymer,drying the fibre and heating to form the oxide of aluminium and todecompose the polymer.
 6. A friction material as claimed in claim 4wherein the fibre is formed by fibrising a composition having aviscosity greater than 1 poise comprising an oxychloride, basic acetate,basic formate or nitrate of zirconium and a minor proportion of awater-soluble organic polymer, drying the fibre and heating to form theoxide of zirconium and to decompose the polymer.
 7. A friction materialas claimed in claim 5 wherein the water-soluble organic polymer ispolyethylene oxide, polyvinyl-alcohol or polyvinylpyrrolidone.
 8. Afriction material as claimed in claim 3 wherein fibrising is effected byextruding the composition through one or more apertures into at leastone converging gas stream having a component of high velocity in thedirection of travel of the extruded composition.
 9. A friction materialas claimed in claim 8 wherein the gas is air at a relative humidity ofgreater than 80 percent.
 10. A friction material as claimed in claim 1wherein the metal oxide fibre has a fibre diameter distribution suchthat the fibres contain not greater than 30 percent by weight of fibresof greater diameter than 5 microns.
 11. A friction material as claimedin claim 4 wherein zirconia fibre is stabilised with a minor proportionof an alkaline earth oxide, yttria, a rare earth oxide, alumina, thoriaor hafnia or mixtures thereof.
 12. A friction material as claimed inclaim 3 wherein the alumina fibre is stabilised with a minor proportionof silica.
 13. A friction material as claimed in claim 3 wherein thealumina is transitional alumina.
 14. A friction material as claimed inclaim 1 wherein the binder is based on a phenol-formaldehyde resin. 15.A friction material as claimed in claim 1 comprising a lubricantselected from litharge, graphite or molybdenum disulphide.
 16. Afriction material as claimed in claim 1 comprising a filler selectedfrom ground barytes, perlite, corundum, borax or a clay.
 17. A frictionmaterial as claimed in claim 1 comprising a metal in the form of wires,turnings or powders selected from zinc, copper, brass, lead or iron. 18.A process for preparing a friction material as claimed in claim 1wherein a synthetic metal oxide fibre is incorporated into a binder andthe mixture shaped into a desired end form.
 19. A process as claimed inclaim 18 wherein the friction material is prepared by wet boardprocessing, comprising the steps of dispersing the fibre, felting thedispersed fibres, saturating the felted fibres in a liquid binder,shaping and solidifying.